The present invention relates to an array of solid polymer electrochemical fuel cell stacks. Typically ion-exchange membranes in such fuel cells must be kept moist to enhance their ionic conductivity and reduce physical degradation resulting in structural failure and leaks. A method and apparatus are provided for utilizing water produced by the electrochemical reaction to keep the ion-exchange membrane moist. More particularly, the present invention improves water management in an array of fuel cell stacks by periodically reversing the reactant flow direction in at least one fuel cell stack.
Electrochemical fuel cells convert fuel and oxidant fluid streams to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. An electrocatalyst is typically disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In typical fuel cells, the MEA is disposed between two electrically conductive separator plates. A fluid flow field provides a means for directing the fuel and oxidant to the respective electrocatalyst layers, specifically, at the anode on the fuel side and at the cathode on the oxidant side. A simple fluid flow field may consist of a chamber open to an adjacent porous electrode layer with a first port serving as a fluid inlet and a second port serving as a fluid outlet. The fluid flow field may be the porous electrode layer itself. More complicated fluid flow fields incorporate at least one fluid channel between the inlet and the outlet for directing the fluid stream in contact with the electrode layer or a guide barrier for controlling the flow path of the reactant through the flow field. The fluid flow field is commonly integrated with the separator plate by locating a plurality of open-faced channels on the faces of the separator plates facing the electrodes. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide structural support for the electrodes.
The fuel stream directed to the anode by the fuel flow field migrates through the porous anode and is oxidized at the anode electrocatalyst layer. The oxidant stream directed to the cathode by the oxidant flow field migrates through the porous cathode and is reduced at the cathode electrocatalyst layer.
Solid polymer fuel cells generally use fuels, such as, for example, hydrogen or methanol, which are oxidized at the anode to produce hydrogen cations. The hydrogen cations migrate through the ion-conducting electrolyte membrane and react with an oxidant such as oxygen in air at the cathode to produce water as one of the reaction products. The anode and cathode reaction equations in hydrogen/oxygen fuel cells are believed to be as follows:
Anode reaction: H2xe2x86x922H++2e31 
Cathode reaction: xc2xdO2+2H++2exe2x86x92H2O
Water in the ion-exchange membrane facilitates the migration of protons from the anode to the cathode. The membrane is electrically non-conductive and also serves as a barrier to separate the hydrogen-containing fuel stream from the oxygen-containing oxidant stream.
The electrons produced at the anode induce an electrical current through an external circuit or load from the anode to the cathode.
Because water is produced by the cathode reaction, as the oxidant stream travels through the oxidant flow field, the oxidant stream absorbs product water. The product water is absorbed as water vapor until the oxidant stream becomes saturated; additional product water may be carried in the oxidant stream as entrained water droplets.
The cumulative effect of product water absorption into the oxidant stream causes the flow field region near the oxidant flow field outlet to contain more water than the flow field region closer to the oxidant stream inlet. Therefore, the fresh oxidant stream typically enters the oxidant flow field at its driest region. If the oxidant stream entering the oxidant flow field is not adequately humidified, the oxidant stream may absorb water from the membrane in the region nearest the oxidant stream inlet.
It is generally well known that most conventional fuel cell ion-exchange membranes must be kept moist to maintain adequate ionic conductivity and to reduce structural damage that may result if the membrane is allowed to become too dry. It is known that leaks in membranes frequently occur near reactant stream inlet ports. Such leaks may be caused or contributed to by inlet streams drying the membrane, resulting in the formation of cracks or holes.
Accordingly, in the prior art, it is known to provide means for keeping the membrane wet and/or humidifying the reactant streams before they enter the flow fields. A disadvantage of conventional methods of humidifying the reactant streams is that incorporating an external humidification apparatus adds to the system complexity and reduces the overall system efficiency.
The production of water at the cathode may cause another problem if too much water accumulates in the oxidant flow field. If the oxidant stream becomes saturated, two phase flow may occur, that is, the oxidant stream may contain water vapor and liquid water droplets. Liquid water in the oxidant flow field can xe2x80x9cfloodxe2x80x9d the porous electrode and obstruct the oxidant from reaching the cathode electrocatalyst. Saturation and flooding is more likely to occur in the portions of the oxidant flow field closest to the outlet, where the oxidant stream has had the most opportunity to accumulate product water.
In view of the above-identified problems, overly wet or dry regions of the flow field can detrimentally affect fuel cell performance and accelerate the degradation of performance over time. Fuel cell performance is defined as the voltage output from the cell for a given current density; higher performance is associated with higher voltage for a given current density. Accordingly, there is a problem with conventional fuel cells that have localized wet and dry regions caused by the cumulative effect of reaction product water absorption into the oxidant stream.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is typically held together in its assembled state by tie rods and end plates.
As with single cell fuel cell assemblies, in a fuel cell stack where fixed inlets and cutlets are typically used for supplying and exhausting reactants, the accumulation of product water in the oxidant flow fields causes similar localized wet and dry conditions in each individual fuel cell of the stack. Consequently, at the inlets near the stack oxidant supply manifolds, the membranes can become overly dry, while the oxidant stream can become saturated near the outlets to the stack oxidant exhaust manifolds.
A fuel cell array typically comprises a plurality of fuel cell stacks that may be electrically connected in series or in parallel. The reactants are typically supplied in parallel to the individual fuel cell stacks in the array. As with an individual fuel cell assembly or a fuel cell stack, water management issues are encountered in conventional fuel cell arrays.
Accordingly, it is an object of the present invention to provide a method and apparatus for managing product water and distributing water to an ion-exchange membrane in an electrochemical fuel cell, which may be one of a plurality of fuel cells in a fuel cell stack or a fuel cell array.
In the present method, water accumulating in an oxidant stream is distributed in an array comprising a plurality of electrochemical fuel cell stacks. The plurality of fuel cell stacks each comprise at least one fuel cell assembly comprising an ion-exchange membrane interposed between a cathode and an anode. Each fuel cell assembly preferably further comprises:
an oxidant flow field associated with the cathode for directing the oxidant stream to the cathode between a first oxidant flow field port and a second oxidant flow field port; and
a fuel flow field associated with the anode for directing a fuel stream to the anode between a fuel stream inlet port and a fuel stream outlet port.
In one embodiment, the method comprises utilizing water accumulating in an oxidant stream flowing through the plurality of fuel cell stacks by periodically reversing the oxidant stream flow direction within oxidant passages within at least one, and preferably each, of the plurality of fuel cell stacks in the array. Preferably, the oxidant stream flow direction is periodically reversed in each of the stacks in the array.
Reversing the flow direction of a reactant within a fuel cell stack may cause certain transition effects. In some circumstances (for example, during operation on dilute reactant streams) reversing the reactant flow direction may cause a small amount of depleted reactant stream to be directed back through the reactant passages causing a momentary decrease in power output, that is, until fresh reactant entering the fuel cell stack is introduced to the passages. Several methods may be employed to reduce such transition effects. For example, it is desirable to locate the flow switching device as close as possible to the fuel cell reactant ports to reduce the amount of depleted reactant that is re-directed back through the fuel cell stack when the reactant flow direction is reversed. It is also desirable for the flow switching device to employ valves that may be quickly and precisely controlled. Another approach is to increase the reactant stoichiometry immediately prior to reversing the reactant stream flow direction.
To further reduce transition effects resulting from reversing the oxidant stream flow direction, preferably the oxidant stream flow direction in an array is controlled so that the flow direction within every one of the plurality of stacks is not reversed simultaneously. That is so that there is at least one stack that is not undergoing oxidant stream flow reversal at any given moment. The oxidant stream flow direction within each one of the plurality of fuel cell stacks may be periodically sequentially reversed at staggered intervals. For example, the flow direction reversals may be sequenced so that the oxidant stream flow direction is not reversed simultaneously in any pair of said plurality of fuel cell stacks. By controlling the relative timing of the flow reversals in stacks in the array, the transition effects are reduced.
An additional method of reducing the impact of transition effects is to control the periodic reversals in the oxidant stream flow direction so that they occur when power output of the array is below a threshold value. In this way, transition effects do not limit power output when peak power output is demanded.
In a preferred embodiment, the disclosed method further comprises controlling the temperature profile within the oxidant stream so that the oxidant stream temperature generally increases in the flow direction. This aspect of the method may be advantageously applied to an array of fuel cell stacks, or to an individual fuel cell stack or fuel cell assembly, that is not part of an array. An advantage of controlling the oxidant stream temperature so that the oxidant stream is cooler where it enters the fuel cell, compared to where it exits the fuel cell is improved water management. As the oxidant stream flows through the fuel cell it is exposed to more product water. Accordingly, if the oxidant stream temperature also rises as it flows through the fuel cell, the water carrying capacity of the oxidant stream also increases. Therefore, when the flow direction of the oxidant stream is periodically reversed, it is advantageous to correspondingly adjust the cooling system so that the temperature profile in the oxidant stream generally increases in the flow direction. In addition to improved handling of product water, reducing the temperature and water carrying capacity of the oxidant stream near the oxidant inlet also helps to prevent membrane dehydration by preventing the oxidant stream from carrying too much water away from the membrane near the oxidant inlet.
In one embodiment, to assist in controlling the temperature profile of the oxidant stream, the coolant fluid flow direction is also controlled so that it is substantially concurrent with the oxidant stream flow direction. Accordingly, a coolant fluid flow field is provided which has inlet/outlet ports which are in the same general location relative to the MEA as the oxidant stream inlet/outlet ports; the coolant fluid flow direction through the coolant flow field is periodically reversed substantially simultaneously with the change in oxidant stream flow direction so that the oxidant and coolant streams flow in substantially the same direction within a fuel cell assembly.
In another preferred embodiment, the method further comprises releasably capturing water from the oxidant stream downstream of at least one of the plurality of fuel cell stacks in the array. At least a portion of the captured water is released back into the oxidant stream upstream of at least one of the plurality of fuel cell stacks. For example, when the oxidant stream flow direction is reversed, the oxidant flow field port previously serving as the outlet becomes the inlet, and at least a portion of the captured water is released into the fresh oxidant stream that is introduced through the port/inlet into the oxidant flow field.
To further improve water management within the array, in a preferred embodiment, the method further comprises periodically reversing the flow direction of a fuel stream substantially simultaneously with the flow direction reversals of the oxidant stream that flows on the opposite side of the same fuel cell membrane. The fuel stream flow direction is reversed to prevent the fuel stream flow direction from being substantially concurrent with the oxidant stream flow direction. The advantage of this technique is that it prevents the oxidant and fuel streams from entering the fuel cell in the same general location with respect to the membrane. When the oxidant and/or fuel supply streams are not adequately humidified, preventing the oxidant and fuel streams from flowing in a substantially concurrent direction reduces membrane drying effects near the oxidant and fuel supply stream inlets by keeping such inlet locations spaced apart. Therefore, in this embodiment, the fuel stream is preferably directed to flow in a direction substantially opposite to the oxidant stream flow direction. Advantageously, the portion of the oxidant flow field near the oxidant outlet, that is typically the most humidified portion of the oxidant flow field, is opposite the portion of the fuel flow field near the fuel inlet, thereby offsetting membrane drying that may result from the introduction of an inadequately humidified fuel stream.
Also provided is a method of distributing water in an array comprising a plurality of electrochemical fuel cell stacks utilizing water in a fuel stream flowing through the plurality of fuel cell stacks. In this embodiment, the fuel stream flow direction is periodically reversed within fuel passages within at least one, and preferably each, of the plurality of fuel cell stacks. Since water is not a product of the induced anode-side reaction, the fuel supply stream is preferably humidified prior to being introduced into a fuel cell assembly. In this method, some water may also migrate from the cathode to the anode, supplementing any humidification water present in the fuel stream. The fuel stream flow direction is periodically reversed for better distribution of the humidification water and/or the migration water to the fuel cell membrane. A fuel flow field associated with the anode may be employed to direct the fuel stream to the anode between a first fuel flow field port and a second fuel flow field port.
Conventional fuel cell assemblies, fuel cell stacks, and arrays have not utilized the fuel stream to distribute water to the fuel cell membrane because efforts relating to solving water management problems have generally been directed to the cathode side where water is a product of the induced reactions, and it is on the cathode side where water xe2x80x9cfloodingxe2x80x9d is typically observed. Some conventional fuel cell systems have humidified the fuel stream prior to introducing it to a fuel cell assembly, but it has not been suggested to reverse the fuel stream flow direction in a fuel cell assembly to improve the distribution of water to the membrane. Reversing the fuel stream flow direction is a technique which may be employed alone or in combination with other techniques, including those described herein, to reduce or eliminate the humidification pretreatment requirements for one or both of the oxidant and fuel streams supplied to a fuel cell assembly.
As with the oxidant stream, it is desirable to reduce the impact of transition effects caused by reversing the flow direction of a reactant stream. Again, preferably the fuel stream flow direction in an array is controlled so that the flow direction within every one of the plurality of stacks is not reversed simultaneously. That is so that there is at least one stack that is not undergoing fuel stream flow reversal at any given moment. The fuel stream flow direction within each one of the plurality of fuel cell stacks may be periodically sequentially reversed at staggered intervals. For example, the flow direction reversals may be sequenced so that the fuel stream flow direction is not reversed simultaneously in any pair of said plurality of fuel cell stacks. Other techniques described above which may be used to reduce the transition effect caused by reversing the oxidant stream may also be adapted to reduce the transition effects when reversing the fuel stream. For example, the periodic reversals in the fuel stream flow direction may be controlled so that they occur when power output of the array is below a threshold value.
The method may further comprise periodically reversing the flow direction of a coolant fluid flowing through at least one, and preferably each, of the plurality of fuel cell stacks. The coolant flow direction is preferably controlled so that the reactant stream that is the primary carrier of membrane hydration water has a temperature profile that generally increases in the direction of flow.
In a single fuel cell or fuel cell stack, the flow direction of a substantially poison-free fuel stream through the fuel cell(s) may be advantageously periodically reversed, to distribute water to an ion-exchange membrane in the fuel cell. Preferably the fuel supply stream is humidified prior to being introduced into the fuel cell, so that a source of the membrane hydration water is humidification water as well as water which has migrated through the membrane from the cathode to the anode. As with the other embodiments, this method may further comprise periodically reversing the flow direction of a coolant fluid flowing through the fuel cell.
Also provided is an electrochemical fuel cell assembly comprising an ion-exchange membrane interposed between a cathode and an anode. The assembly further comprises:
an oxidant flow field associated with the cathode for directing an oxidant stream to the cathode between a first oxidant flow field port and a second oxidant flow field port;
a fuel flow field associated with the anode for directing a fuel stream to the anode between a fuel stream inlet port and a fuel stream outlet port;
a coolant system comprising at least one coolant passage associated with the fuel cell for receiving a coolant fluid which flows through the at least one coolant passage;
an oxidant stream flow switching device for periodically reversing the direction of flow of the oxidant stream between the first and second oxidant flow field ports; and
a coolant fluid flow switching device for periodically reversing the direction of flow of the coolant fluid through the at least one coolant passage.
Such an electrochemical fuel cell assembly preferably further comprises a controller for operating the oxidant flow switching device and the coolant fluid flow switching device such that the oxidant stream flows through the fuel cell substantially concurrently with the coolant fluid.
Another embodiment of the apparatus provides a fuel cell array comprising a plurality of electrochemical fuel cell stacks. Each one of the plurality of fuel cell stacks comprises at least one fuel cell assembly. This embodiment further comprises an oxidant stream flow switching device for periodically individually reversing the oxidant stream flow direction within at least one, and preferably each, of the plurality of fuel cell stacks without simultaneously reversing the oxidant stream flow direction in all of the plurality of fuel cell stacks.
In accordance with this embodiment, each one of the at least one fuel cell assembly preferably comprises:
an ion-exchange membrane interposed between a cathode and an anode;
an oxidant flow field associated with the cathode for directing an oxidant stream to the cathode between a first oxidant flow field port and a second oxidant flow field port; and
a fuel flow field associated with the anode for directing a fuel stream to the anode between a fuel stream inlet port and a fuel stream outlet port.
The fuel cell array preferably further comprises an oxidant manifold for supplying the oxidant stream to each of the plurality of fuel cell stacks in parallel.
Preferably the fuel cell array further comprises an oxidant stream flow switching device that reverses the oxidant stream flow direction in a staggered sequence so that the oxidant stream flow direction is not reversed simultaneously in any pair of the plurality of fuel cell stacks.
In the alternative, the fuel cell array of further comprises a controller for actuating the oxidant stream flow switching device and controlling the timing for periodically reversing the oxidant stream flow direction in each of the plurality of stacks.
The controller preferably adjusts the timing for periodically reversing the oxidant stream flow direction responsive to a measured operational parameter of the array. For example, the controller may preferably actuate the oxidant stream flow switching device to reverse the oxidant stream flow direction in at least one, and preferably each, of the plurality of fuel cell stacks when the power output of the fuel cell array is below a predetermined threshold value.
The fuel cell array may further comprise a water recycler associated with the first and second oxidant flow field ports of at least one of the plurality of fuel cell stacks. The water recycler releasably captures water from an exhaust oxidant stream and releases at least a portion of the captured water into an oxidant stream upstream of at least one of the plurality of fuel cell stacks. The water recycler preferably comprises a hygroscopic media.
As with a single fuel cell stack, a fuel cell array preferably further comprises a cooling system comprising cooling fluid passages located within the plurality of fuel cell stacks and a cooling fluid flow switching device for periodically reversing the cooling fluid flow direction within the cooling fluid passages. A cooling fluid controller is preferably employed for synchronizing changes in cooling fluid flow direction with changes in oxidant stream flow direction. The cooling fluid controller may be used to control the cooling fluid flow direction so that it is substantially concurrent with the oxidant stream flow direction. In this way, the temperature profile in the oxidant stream generally increases in the flow direction.
In yet another embodiment, a fuel cell array comprises a plurality of electrochemical fuel cell stacks, wherein each one of the plurality of fuel cell stacks comprises at least one fuel cell assembly. The apparatus further comprises a fuel stream flow switching device for periodically individually reversing the flow direction of a fuel stream within at least one, and preferably each, of the plurality of fuel cell stacks without simultaneously reversing the fuel stream flow direction in all of the plurality of fuel cell stacks. In operation, preferably the fuel stream is humidified and is substantially poison-free. In this context, xe2x80x9cpoisonsxe2x80x9d are defined as components within a fuel stream that may adversely affect the performance of the anode catalyst. For example, carbon monoxide is a well-known poison for most conventional anode catalysts.
In this embodiment, each one of the at least one fuel cell assembly preferably comprises:
an ion-exchange membrane interposed between a cathode and an anode;
a fuel flow field associated with the anode for directing a fuel stream to the anode between a first fuel flow field port and a second fuel flow field port; and
an oxidant flow field associated with the cathode for directing an oxidant stream to the cathode between an oxidant stream inlet port and an oxidant stream outlet port.
Preferably, the fuel stream flow switching device reverses the fuel stream flow direction in a staggered sequence so that the fuel stream flow direction is not reversed simultaneously in any pair of the plurality of fuel cell stacks.
In this embodiment, the fuel cell array preferably further comprises a controller for actuating the fuel stream flow switching device and controlling the timing for periodically reversing the fuel stream flow direction in each of the plurality of stacks. The controller may further adjust the timing for periodically reversing the fuel stream flow direction responsive to a measured operational parameter of the array. For example, the controller may be responsive to an electrical output of the fuel cell array. The controller preferably actuates the fuel stream flow switching device to reverse the fuel stream flow direction in at least one, and preferably each, of the plurality of fuel cell stacks when the power output of the fuel cell array is below a threshold value.
The fuel cell array may further comprise a water recycler associated with the first and second fuel flow field ports of at least one of the plurality of fuel cell stacks. The water recycler releasably captures water from an exhaust oxidant stream and releases at least a portion of the captured water into a fuel stream upstream of at least one of the plurality of fuel cell stacks. In one embodiment, the water recycler comprises a hygroscopic media.
This embodiment of the fuel cell array of claim may additionally comprise a cooling system comprising cooling fluid passages located within the plurality of fuel cell stacks and a cooling fluid flow switching device for periodically reversing the cooling fluid flow direction within the cooling fluid passages. A cooling fluid controller may be employed for synchronizing changes in cooling fluid flow direction with changes in fuel stream flow direction.